1. Technical Field
The present invention relates to an imaging lens that forms an optical image of a subject on an imaging device such as a CCD (charge coupled device) or a CMOS (complementary metal oxide semiconductor) and to an imaging apparatus such as a digital still camera, a portable cellular phone, and a PDA (personal digital assistance) provided with the imaging lens and performing image taking.
2. Related Art
In recent years, with the spread of personal computers to ordinary households and the like, digital still cameras capable of inputting image information of taken images of landscapes, persons and the like to personal computers have been rapidly spreading. Moreover, more and more portable cellular phones have been provided with a camera module for image input. For apparatuses having such an imaging function, an imaging device such as a CCD or a CMOS is used. In recent years, these imaging devices have been made more and more compact, and the whole size of imaging apparatuses and the size of imaging lenses provided in the imaging apparatuses are required to be small. At the same time, the number of pixels of imaging devices has been increasing, and imaging lenses are required to have higher resolution and higher performance. For example, performance supporting as high as two mega-pixels or more, more suitably, five mega-pixels or more is required.
To satisfy these requests, an imaging lens of a structure using four lenses as a whole has been developed (see Patent Document 1 (JP-A-2002-228922 corresponding to US 2002/0181121), Patent Document 2 (JP-A-2004-053813 corresponding to US 2006/0132933) and Patent Document 3 (JP-A-2004-102234 corresponding to US 2004/0012861)).
Imaging lens having four lenses has been known in which the aperture diaphragm is disposed on the most object side (see Patent Document 1). However, as the angle of view is made wider, when the aperture diaphragm is disposed on the most object side, the performance degradation sensitivity to the manufacturing error and the lens surface position shift are apt to increase, and the manufacturability tends to be degraded. On the other hand, Patent Documents 2 and 3 disclose imaging lenses in which the aperture diaphragm is disposed between a first lens and a second lens. However, the power of the first lens is relatively high in the lens system, and if the angle of view is widened, the performance of, particularly, the field curvature is degraded, which makes the lens system insufficient in performance. It is demanded that a lens be developed that has high performance, for example, performance supporting as high as two mega-pixels or more, more desirably, approximately five to ten mega-pixels even though the angle of view is widened.
The present invention is made in view of such a problem, and an object thereof is to provide an imaging lens in which the angle of view is easily widened and a lens system with excellent manufacturability can be realized and an imaging apparatus provided with the imaging lens and capable of obtaining high-resolution taken images.